1. Field of the Invention
This invention relates to a thermal energy storage tank utilizing a phase changeable thermal energy storage material, and more particularly to improvements in the construction thereof.
2. Description of the Prior Art
A phase changeable material or meltable material (This will be referred to as a thermal energy storage material, as the case may be.) presents a large amount of latent heat upon shifting of one phase to another i.e., from liquid to solid (solidification) or from solid to liquid (liquefaction). Stated differently, the above material may retain a large heat capacity without being attended with a temperature change. The above property of the phase changeable material is used for the purposes of thermal energy storage or coldness preservation (This may be regarded as thermal energy storage in the sense of the accumulation of negative heat. However, thermal energy storage and coldness-preservation will be dicriminated in use, except in connection with a thermal energy storage tank).
A system which needs thermal energy storage is a solar-thermal electric power plant. In this case, it is preferable to use as a thermal energy storage material a meltable material having a high melting temperature and a large amount of latent heat, such as eutectic salts KF--LiF--NaF (Mol % . . . 42 to 46.5 - 11.5; melting temperature . . . 454.degree. C; latent heat . . . 95 kcal/kg) for compensating for a time-inconformity between an insolation time zone and a loading time zone.
On the other hand, as an example of coldness-preservation, water is used as a coldness-preserving material by freezing same by using excess electric power, so that the coldness of ice thus produced may be utilized for cooling during the day time.
A prior art thermal energy storage tank, for instance, a solar thermal electric power plant will be described in more detail with reference to FIG. 1.
Referring to FIG. 1, shown at 2 a heat exchange tube, at 4 a lower supporting disc, at 3 an upper supporting disc, at 5 a heat transfer medium inlet tube, at 6 a heat transfer medium outlet tube, at 8 a vessel covered with a heat insulating wall or layer 7 from externally, and at 9 a thermal energy storage material.
Thus, in case heat-collecting working water (This will be referred to as supply water hereinafter, as the case may be.) may be heated to a level to produce steam of a sufficiently high temperature due to satisfactory insolating condition, high temperature steam, as a heat transmitting medium, flows via an inlet tube 5 into the thermal energy storage tank 1, and passes through heat exchange tubes 2, where the steam imparts its latent heat to the thermal energy storage material 9 (This has remained in a solid state initially. This will be referred to as a thermal energy storage solid material 9A, hereinafter, as the case may be.) for accumulating latent heat therein.
Meanwhile, after imparting the aforesaid latent heat, steam will not be lowered to a temperature below the melting temperature of the thermal energy storage material in a normal condition, so that steam may be supplied, as it is, to a loading system after being discharged from the outlet tube 6.
On the other hand, in case supply water is not heated to a sufficient level due to an unsatisfactory insolation condition, steam of a relatively low temperature or water is introduced, as it is, through the heat exchange tubes 2 in the thermal energy storage tank 1, so that the supply water or steam of a relatively low temperature may be supplied with latent heat from the aforesaid thermal energy storage material 9 (This in general remains in a liquid state, and will be referred to as a thermal energy storage liquid material 9B, hereinafter, as the case may be.), so that water or steam may be heated to the melting temperature of the thermal energy storage material 9, and then supplied to a loading system as a high temperature steam.
However, the prior art thermal energy storage tank is found to be defective in its construction, because of a drawback arising from a volumetric change due to shifting from a solid phase to a liquid phase of the thermal energy storage material, and vice versa.
For instance, KF--LiF--NaF eutectic salt exhibits a volumetric increase of about 20%, when shifting from a solid phase to a liquid phase. Other thermal energy storage materials provide a tendency similar thereto.
The above drawback will be described in more detail with reference to the operation of the aforesaid thermal energy storage tank. The thermal energy storage material begins melting around the entrance of the heat exchange tube 2 for a heat transfer medium, i.e., in the neighborhood of a connecting portion between the heat exchange tube 2 and the lower supporting disc 4 (Refer to 9B). In this respect, an volumetric increase due to melting has been suppressed by unmelted thermal energy storage material 9A positioned above, so that a high stress is imposed on the heat exchange tube 2, lower supporting disc 4 and connecting portion 11 between the heat exchange tube and the lower supporting disc, thus damaging the thermal energy storage tank, if the case comes to the worst.
For avoiding the above drawback, it may be a solution to reverse the inflow direction of a heat collecting supply water, i.e., the direction from above to below, as viewed in FIG. 1.
In this case, the thermal energy storage material begins melting of an upper portion of the thermal energy storage tank and is retained in an upper space portion (gas layer) 10 of the vessel, thus avoiding the aforesaid drawback.
However, this attempt brings about another new problem that, during heat-removing operation, a heat transfer medium, i.e., steam of a relatively low temperature, or particularly water can not be distributed for respective heat exchange tubes uniformly.
In other words, when the flow rates of supply water through the heat exchange tubes 2 become lack of uniformity, then fluidic resistance of a heat exchange tube allowing an increased flow rate of water is reduced (because of faster cooling), so that the flow rate of supply water is further increased, resulting in an increased uneveness in flow rate of supply water for respective heat exchange tubes.
Attempts for avoiding this drawback are to provide an orifice resistance for the entrance of a heat exchange tube and to reduce the diameter of a heat exchange tube for increasing the flow speed of supply water, thereby improving the distribution of flow rates for respective heat exchange tubes.
However, the former attempt suffers from a disadvantage in that the supply water flows down along a localized surface of a heat exchange tube downstream of an orifice, thus failing to derived steam of a sufficiently high temperature, as in the preceding case. On the other hand, the latter attempt poses a disadvantage in that the length of heat exchange tubes should be increased excessively, for providing a desired heat exchange surface, thus resulting in an increase in pressure loss in a heat transfer medium system.
Description has been given thus far of heat exchange tubes of a linear form, with reference to FIG. 1. However, heat exchange tubes other than the linear tubes, i.e., spiral tubes or zig-zag tubes may be used, with the same shortcomings attended.